1. Field of the Invention
The invention relates generally to a system and method for a strategy for monitoring and controlling engine NOx emissions, and more particularly to a system and method for monitoring and controlling NOx emissions during restarts of hybrid and conventional vehicles.
2. Background Art
Hybrid electric vehicles (HEVs) utilize a combination of an internal combustion engine (ICE) in connection with an electric motor. The electric motor is fed electrical energy from a battery to provide the power needed to propel a vehicle (i.e., provide power to vehicle wheels) for improved fuel economy when compared to a conventional (i.e., an internal combustion engine only) vehicle. In a hybrid vehicle, fuel economy can be improved and exhaust emissions can be reduced by shutting down (i.e., turning off) the engine, especially when the ICE operates inefficiently, and using the electric motor to provide all of the power needed to propel the vehicle. As such, hybrid electric vehicles (HEVs) typically perform engine shut off and restart operations frequently while driving.
However, when the user (e.g., driver of the vehicle) wants more power than the electric motor can provide or when the battery becomes depleted, the engine is generally restarted to provide the requested additional power. During engine shut downs and restarts, air is pumped by the engine into the exhaust system. An exhaust system catalyst adsorbs the oxygen from the air. Frequently, the volume of the adsorbed oxygen is sufficient to saturate the oxygen storage capacity of the catalyst. A three-way catalyst (TWC) that has a saturated or nearly saturated oxygen storage capacity can have diminished NOx conversion ability. When combustion is resumed (i.e., when the engine is restarted), the NOx emissions from the engine typically are not efficiently reduced or eliminated by the catalytic converter until the oxygen storage level in the catalytic converter is reduced from the saturated level.
Conventional non-hybrid vehicles (i.e., vehicles that are powered only be an internal combustion engine) operate under similar circumstances during deceleration fuel shutoff. As such, conventional vehicles can have diminished NOx conversion ability during deceleration fuel shutoff.
One example of a conventional approach to controlling exhaust emissions for ICEs is disclosed in U.S. Pat. No. 6,629,408. The exhaust emission control system includes first and second oxygen concentration sensors respectively provided upstream and downstream of the nitrogen oxide removing device for detecting an oxygen concentration in the exhaust gases. The air-fuel ratio of an air-fuel mixture to be supplied to the engine is changed from a lean region to a rich region with respect to a stoichio-metric ratio. An amount of reducing components flowing into the nitrogen oxide removing device from the time when an output value from the first oxygen concentration sensor has changed to a value indicative of a rich air-fuel ratio after enrichment of the air-fuel ratio is calculated. Deterioration of the nitrogen oxide removing device is determined according to the calculated amount of reducing components and an output value from the second oxygen concentration sensor. However, such conventional approaches generally fail to monitor and predict the effectiveness of the HEV and conventional ICE vehicle restart emissions performance, and fail to provide remedial processes.
Thus, to efficiently and effectively provide for controlling exhaust emissions for ICEs, there is a need for an improved system and method for monitoring NOx emissions during restarts of hybrid vehicles and conventional vehicles.